参考表 
count()用来统计记录的条数
mysql> select count(*) from score;
mysql> select count(*) from score where stu_id=901;
sum()求和函数
mysql> select sum(grade) from score;
mysql> select id,sum(grade) from score where stu_id=901;
avg()求平均值函数
mysql> select avg(grade) from score where c_name='计算机';
mysql> select c_name,avg(grade) from score group by c_name;
max()求最大值函数
mysql> select c_name,max(grade) from score where c_name='英语';
mysql> select c_name,max(grade) from score group by c_name;
min()求最小值函数
mysql> select c_name,min(grade) from score where c_name='中文';
mysql> select c_name,min(grade) from score group by c_name;
Concat拼接函数
mysql> select Concat(c_name, '(', stu_id, ')')
-> from score order by stu_id;
